1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser.
2. Description of the Related Art
Known surface emitting lasers include vertical cavity surface emitting lasers prepared by sandwiching an active region at opposite sides thereof between two reflectors and forming an optical cavity in the direction perpendicular to the substrate surface so as to emit a beam in the direction perpendicular to the substrate surface.
Research efforts have been intensively paid on vertical cavity surface emitting lasers since the late 1980s because those lasers provide a number of advantages as listed below.
This type of surface emitting laser shows a low threshold and a low power consumption rate and gives rises to a circular spot profile. Additionally, it can be coupled with an optical element with ease and arranged to form an array.
However, since this type of surface emitting laser has a small gain region, the pair of distributed Bragg reflectors (to be referred as DBRs hereinafter) that produce a cavity are required to show a reflectivity not lower than 99%.
In the case of semiconductor reflectors, they are required to be formed from a multilayer film having tens of layers in order to realize such reflectors. Then, because of this large thickness of the multilayer film, heat can be trapped in the cavity and the cavity tends to show a large threshold and/or a large electric resistance and hence current injection becomes difficult.
Several proposals have been made to date for cavity reflectors that can replace such DBRs).
For example, V. Lousse, et al., “Angular and polarization properties of a photonic crystal slab mirror”, Optics Express, Vol. 12, No. 8, pp. 1575-1582 (Apr. 19, 2004) reports the wavelength dependency of reflected light and transmitted light when a two-dimensional slab photonic crystal is used as reflector.
A photonic crystal is a structure of a material that is artificially provided with refractive index modulation of about the wavelength of light. In other words, it is a structure where mediums having mutually different respective refractive indexes are disposed periodically. With a photonic crystal, it is believed that the propagation of light in crystal can be controlled due to the multiple scattering effect of light.
According to the above V. Lousse et al article, it is possible to have a two-dimensional slab photonic crystal reflect light substantially with an efficiency of 100% when the photonic crystal is formed by periodically arranging holes in a slab material showing a high refractive index.
It is reported that, when light is made to strike a so-called hole type two-dimensional slab photonic crystal in a direction substantially perpendicular to the crystal plane, light showing a predetermined frequency is reflected substantially with an efficiency of 100%.
A reflector that has been conventionally formed by a multilayer film with a thickness of several micrometers can now be formed by a very thin film having a thickness of tens to hundreds of several nanometers as a photonic crystal reflector to be used as reflector in a vertical cavity surface emitting laser.
Then, as a result, it is possible to significantly alleviate the problems of a thick reflector such as the difficulty of discharging heat and a high electric resistance.